This invention relates to a mobile radio communication system which simultaneously covers an area having a large number of mobile subscribers and an area having a small number of mobile subscribers, and more particularly to an economical control channel selection system.
To cover a large number of mobile subscribers, many radio channels are required. For example, in a land mobile telephone system presently under consideration, the use of 1,000 channels is planned. For the most efficient use of these radio channels, it is desired that each of the mobile units have the function of selectively using all of the radio channels. In practice, however, since this is technically impossible and seriously disadvantageous from the economical point of view, it has been proposed to divide the mobile subscribers into a plurality of groups and employ different channels for the respective groups. Further, in a mobile radio communication system employing a large number of radio channels, one radio channel is fixedly determined as a control channel and the other radio channels are used as speech channels, and the mobile units are always tuned to the control channel when in the waiting state. Each mobile unit is called with high-speed digital signals through the control channel, and speech channel assignment, etc., are achieved, thereby permitting the mobile unit to communicate with a base station through one idle speech channel assigned by the base station. Where the mobile units are divided into a plurality of groups, the control channel differs with the groups. Since the mobile subscribers move in and out of large and small cities, it is necessary that a base station in an area having a small number of mobile subscribers have equipment of the same scale as in a mobile station in larger cities in connection with the control channel so as to enable transmission in, and reception from, the control channels of the respective groups. For example, in the case of four groups of mobile subscribers, four control channels are needed. Also, in an area where the number of mobile subscribers is small and hence traffic density is so low that about two to ten speech channels are sufficient, the base station is still required to be provided with equipment for four control channels. Accordingly, the conventional system is economically disadvantageous.